Fluid catalytic cracking (FCC) is a unit operation in which petroleum fractions of higher molecular weight are cracked into smaller molecules under heat and with a catalyst. During the cracking process, coke deposits form on the surface of the catalyst, necessitating regeneration of the catalyst. Therefore, the catalyst is continuously separated from the vapors generated by the cracking process and regenerated in a FCC regenerator where the coke deposits are burned off and the catalyst activity is restored.
The FCC regenerator can operate in two modes: full burn and partial burn. In the full burn mode, most of the carbon in the coke deposits is converted to CO2 by reacting with oxygen in the oxidant stream that is also fed to the regenerator. When the regenerator is operated in the partial burn mode, the carbon reacts with oxygen in the oxidant stream and is converted to both CO and CO2. In this instance, the CO in the regenerator flue gas is typically oxidized to CO2 in a downstream boiler to recover heat from the CO oxidation and also to limit emissions of CO in the boiler flue gas. The CO boiler has air fired burners to create a hot flame zone that the regenerator flue gas has to pass through in which the CO is oxidized to CO2. Refinery off-gas can be used as auxiliary fuel for the CO boiler burners. The heat released by the oxidation of CO and from the combustion of the refinery gas is recovered in the boiler to produce process steam. The FCC regenerator flue gas also contains other trace species such as SO2, NOx, and species of reduced nitrogen such as NH3 and HCN. Typically, most of the nitrogen in the carbon deposits is oxidized to NOx in the full burn mode. In the partial burn mode, some of the nitrogen is also transformed to NH3 and HCN, and some of the NH3 and HCN is oxidized to NOx in the downstream CO boiler. The amount of NOx plus the amount of other reduced nitrogen species such as HCN, NH3, CN, HNO is conveniently called “total fixed nitrogen” (or “TFN)” hereafter.
The most common mode of regenerator operation currently in use is the full burn mode. Recently, interest has been renewed in the partial burn mode because of the refiner's desire to maximize FCC production capacity, but there are technical limits in terms of how much feed one can push through a FCC unit in a given time. For example, when the feed rate to a FCC is increased the FCC regenerator flue gas will contain more CO if the FCC is already operating at maximum air blower limitations. This increased CO in the FCC regenerator flue gas must be burned in the downstream CO boiler to meet environmental regulations. For some boilers this may present a problem because the boiler may not be able to destroy the increased CO down to the ppm (parts per million) levels required for compliance with environmental regulations. Thus, the capability of the boiler to destroy CO becomes a bottleneck for any upstream FCC capacity improvement measures.
The total firing rate of the CO boiler burners is largely dictated by the need to provide a flame temperature high enough for sufficient burnout of CO in the FCC regenerator flue gas. Typically flame temperatures of about 1800 F are recommended although the auto ignition temperature of the CO gas is much lower (about 1450 F). For a given regenerator flue gas composition, there is a minimum boiler firing rate below which the amount of thermal destruction of CO that is achieved is not satisfactory. In many occasions, this minimum boiler firing rate produces excess process steam which is ultimately vented to the ambient atmosphere without any use. This represents a waste of fuel energy.
Some FCC systems have low temperature NOx and/or NOx / SOx removal devices downstream of the CO boiler. The low temperature NOx removal process normally requires a specified amount of gas residence time for achieving the desired NOx reduction efficiency. Another problem associated with the FCC capacity increase is that the volume of the FCC regenerator flue gas may also increase. The increase of the regenerator flue gas volume shortens the gas residence time available for the downstream NOx removal devices and reduces their NOx reduction efficiency. The increase in the regenerator flue gas volume also promotes carryover of corrosive scrubbing fluid and increases the risk of accelerated corrosion after the scrubber.
Other processes that treat FCC regenerator flue gas differ from the present invention, but differ in significant conditions and do not provide the advantages that the present invention achieves. For instance, U.S. Pat. No. 5,240,690 teaches adding oxygen-containing gas to regenerator flue gas to produce an off gas having a temperature between 1000 F and 1600 F, but states that the objective is to increase the formation of NOx in the flue gas. U.S. Pat. No. 5,716,514 discloses a method in which carbon monoxide is preferentially not converted to carbon dioxide. U.S. Pat. No. 5,830,346 discloses a method that requires use of a catalyst for the conversion.